Control circuit for the power controlled operation of a load

ABSTRACT

To improve a control circuit for the power-controlled operation of a load, comprising a semiconductor switch which is effective in a load circuit and comprising a drive circuit for the semiconductor switch, which generates a control signal, comprising drive pulses following one after the other and separated by interpulse periods, for controlling the said semiconductor switch in a part-load range, in such a way that as little heat as possible is generated, it is proposed that the control signal should generate in an upper part-load range a first pulse signal, with first individual pulses following one another with a first pulse frequency, and a second pulse signal, with second individual pulses following one another with a second frequency, in the interpulse periods of the first pulse signal, and that the second frequency should be greater than the first frequency by at least a factor of 10.

[0001] The invention relates to a control circuit for thepower-controlled operation of a load, comprising a semiconductor switchwhich is effective in a load circuit and comprising a drive circuit forthe semiconductor switch, which generates a control signal, comprisingdrive pulses following one after the other and separated by interpulseperiods, for controlling the said semiconductor switch in a part-loadrange.

[0002] A control circuit of this type is known from the prior art, forexample DE 197 02 949 A1.

[0003] A control circuit of this type generates a control signal whichhas drive pulses which follow one another in a defined frequency and arepulse-width-modulated for controlling the load.

[0004] Since the switching on and off of the load brought about by thedrive pulses leads in a known way to high losses, and in particularincreased heat generation, especially in the case of an inductive loadwith a freewheeling element, i.e., for example, a freewheelingsemiconductor component, it is to be regarded as the object of thepresent invention to improve a control circuit of the generic type insuch a way that as little heat as possible is generated.

[0005] This object is achieved according to the invention in the case ofa control circuit of the type described at the beginning by the controlsignal generating in an upper part-load range a first pulse signal, withfirst individual pulses following one another with a first pulsefrequency, and a second pulse signal, with second individual pulsesfollowing one another with a second frequency, in the interpulse periodsof the first pulse signal, and by the second frequency being greaterthan the first frequency by at least a factor of 10.

[0006] It is consequently to be regarded as the essence of the presentinvention on the one hand to drive the load, i.e. for example a motoracting as an inductive load, by the first pulse signal with lowfrequency in such a way that this pulse signal allows as much power aspossible to be supplied to the load with as few switching-on andswitching-off operations as possible, but on the other hand likewise tosupply power by the second pulse signal with high frequency in the longperiods between the individual pulses caused by the low frequency of thefirst pulse signal, in order to avoid the disadvantages of exclusivelyoperating the load by only the first pulse signal with low frequency,these disadvantages being manifested in particular by the development ofmechanical noises or the production of mechanical resonances.

[0007] With regard to controlling the power which is to be supplied tothe load, a wide variety of possibilities exist here. One possibilitywould be to alter the frequency of the second pulse signal or, ifappropriate, also of the first pulse signal.

[0008] Since, however, defined fixed frequencies are preferably used forthe pulsed operation of a load to avoid peripheral disturbinginfluences, it is preferably provided that in the upper part-load rangeat least one of the first and second pulse signals can bepulse-width-modulated for power control, i.e. that the power supplied tothe load can be controlled by means of varying the pulse width while therespective pulse signal has a fixed frequency.

[0009] It is particularly favourable in the case of the solutionaccording to the invention if in the upper part-load range both pulsesignals can be pulse-width-modulated, so that incremental adjustment ofthe pulse width of the two pulse signals allows the desired precision ofthe control to be achieved by setting the pulse width of that pulsesignal which has the smaller increments. This is preferably the secondpulse signal, it also being possible to use the second pulse signal toreduce an increase in the pulse width of the first pulse signal by adecreasing pulse width of the second pulse signal.

[0010] In the case of a control circuit operating in a particularlysimple way, it is preferably provided that in the upper part-load rangeonly one of the pulse signals can be pulse-width-modulated.

[0011] In this case, particularly precise power control can be carriedout by allowing the upper part-load range to be divided into a highestupper part-load range and a normal upper part-load range and by allowingthe individual pulses of the first pulse signal to bepulse-width-modulated in the normal upper part-load range and theindividual pulses of the second pulse signal to be pulse-width-modulatedin the highest upper part-load range.

[0012] In particular in the case of an embodiment operating in as simplea way as possible, it is at the same time provided that in the normalupper part-load range, the pulse width of the individual pulses of thesecond pulse signal is constant, and in the highest upper part-loadrange, the pulse width of the individual pulses of the first pulsesignal is preferably constant.

[0013] It would be conceivable, for example, within the scope of thesolution according to the invention, also to operate outside the upperpart-load range with a control signal which exhibits the first pulsesignal and, in the interpulse periods of the same, the second pulsesignal.

[0014] However, for reasons of control simplicity and adequateprecision, it is particularly favourable if, in a part-load range lyingbelow the upper part-load range, the control signal comprises a thirdpulse signal with a third frequency, which is greater than the firstfrequency, so that the individual pulses of the third pulse signalfollow one another at correspondingly small time intervals.

[0015] At the same time, the third frequency should preferably be of thesame order of magnitude as the second frequency, so that both the secondfrequency and the third frequency are significantly above the firstfrequency, to allow driving of the load to be performed in the lowerpart-load range with as little noise and resonance as possible.

[0016] A solution which is particularly favourable on account of itssimplicity provides that the third frequency and the second frequencyare approximately of the same magnitude, so that the advantageouscontrol properties of a pulse signal with a relatively high pulsefrequency can consequently be utilized both in the lower part-load rangeand in the upper part-load range.

[0017] A particularly favourable solution provides that the thirdfrequency is identical to the second frequency, so that ultimately thesame frequency can always be used both for operating the load in thelower part-load range and for operating the load in the upper part-loadrange, and the second pulse signal is simply added when there is atransition from the lower part-load range to the upper part-load range.

[0018] For the power control in the lower part-load range, in this casethe third pulse signal can preferably likewise be pulse-width-modulated.

[0019] The transition from the lower part-load range into the upperpart-load range may in principle lie at any desired values of the partload. A particularly advantageous solution provides that the transitionfrom the lower part-load range into the upper part-load range takesplace at part-load values between approximately 20% and approximately50%, in each case with respect to full load.

[0020] A particularly favourable solution provides that the transitionfrom the lower part-load range into the upper part-load range takesplace at part-load values of approximately 30% to approximately 40%.

[0021] With regard to the differences of the second frequency and thethird frequency with respect to the first frequency, it is adequate inprinciple—as already stated—that they are at least a factor of 10. It isparticularly favourable, however, if the frequency differences have afactor of the order of magnitude of 30 or more, preferably the order ofmagnitude of 100 or more.

[0022] In principle it would be possible to operate in the part-loadrange below full driving of the load with further part-load ranges, forexample also between the lower part-load range and the upper part-loadrange. For reasons of simplicity, however, it has been found to befavourable if the upper part-load range follows on directly from thelower part-load range.

[0023] It would additionally also be conceivable to provide outside thelower and upper part-load ranges additional part-load ranges, in which adifferent kind of driving of the part load can take place.

[0024] It is particularly favourable, however, if the lower part-loadrange and the upper part-load range cover the entire part-load range upto full load.

[0025] With regard to generating the control signals in the case of acontrol circuit according to the invention, no further details have beenspecified, in particular concerning the construction of the drivecircuit. Thus, an advantageous solution provides that the drive circuithas a pulse generator and a pulse-shaping stage, it being possible forthe pulse generator to be in particular a pulse generator substantiallygenerating square-wave pulses, and the pulse-shaping stage then shapingthe edges of the square-wave pulses for example in such a way thatsufficiently long control times are available for the operation of theload, in particular an inductive load with a freewheeling diode.

[0026] It is particularly favourable in this case if the pulse-shapingstage generates from the square-wave pulses, rise and fall times whichare time-delayed substantially in the edges.

[0027] Time-delayed rise and fall times of this type can be generated,for example, by RC elements of the pulse-shaping stage.

[0028] With regard to the generation of the first pulse signal and thesecond pulse signal, occurring in the interpulse periods of the firstpulse signal, no further details have been specified so far. Thus, anadvantageous embodiment provides that the first pulse signal and thesecond pulse signal can be generated as pulse signal trains havingcontinuous individual pulses with constant frequency and that thecontrol signal for the upper part-load range is produced from the pulsesignal trains by conducting an OR operation.

[0029] This type of control signal generation can also be retained whenthe control signal for the lower part-load range is to be generated. Inthis case, for the sake of simplicity, the pulse width of the firstpulse signal is reduced to substantially 0.

[0030] In addition, the object mentioned at the beginning is alsoachieved according to the invention by a method for the power-controlledoperation of a load by means of a control circuit, comprising asemiconductor switch which is effective in a load circuit and comprisinga drive circuit for the semiconductor switch, which generates a controlsignal, comprising drive pulses following one after the other andseparated by interpulse periods, for controlling the said semiconductorswitch in a part-load range, by the control signal being generated in anupper part-load range in such a way that it has a first pulse signal,with individual pulses following one another with a pulse frequency, anda second pulse signal, with individual pulses following one another witha second frequency, in the interpulse periods of the first pulse signal,and by the second frequency being greater than the first frequency by atleast a factor of 10.

[0031] It is particularly favourable in this case if, in the upperpart-load range, the power control is carried out by pulse widthmodulation of at least one of the first and second pulse signals, thefrequency of the first pulse signal and of the second pulse signalpreferably being kept constant.

[0032] A particularly favourable solution with regard to the possibilityof variation in this case provides that the pulse width of both pulsesignals is modulated for the power control.

[0033] For reasons of simplicity, however, it is favourable if only thepulse width of one of the pulse signals is modulated, whereas the otherof the pulse signals is kept constant.

[0034] Furthermore, it is of advantage for particularly precise controlif the upper part-load range is divided into a highest upper part-loadrange and a normal upper part-load range.

[0035] Preferably, the first pulse signal is modulated with regard tothe pulse width for the power control in the normal upper part-loadrange, while the second pulse signal is modulated for controlling thepower in the highest upper part-load range.

[0036] For the sake of simplicity, the other pulse signal, respectively,in this case remains constant with regard to its pulse width.

[0037] In addition, it is of advantage for controlling the power outsidethe upper part-load range if, below the upper part-load range, a controlsignal which comprises a third pulse signal with a third frequency,which is greater than the first frequency, is generated.

[0038] In this case, the third frequency is preferably likewise chosensuch that it is of the same order of magnitude as the second frequency.

[0039] For reasons of simplicity, however, it is particularly favourableif the third frequency is identical to the second frequency.

[0040] Furthermore, it is likewise of advantage for controlling thepower in the lower part-load range if the third pulse signal ismodulated with regard to its pulse width, the frequency with which theindividual pulses follow one another in the case of the third pulsesignal likewise being kept constant in particular.

[0041] With regard to the position of the upper part-load range and ofthe lower part-load range in relation to one another, so far likewise nospecific details have been specified. The upper part-load range and thelower part-load range could, for example, still be separate from oneanother. It is particularly favourable, however, if the upper part-loadrange follows on directly from the lower part-load range.

[0042] Furthermore, it is preferably provided for reasons of simplicitythat the lower part-load range and the upper part-load range cover theentire part-load range up to full load.

[0043] To allow the control signal to be generated as simply as possiblein the case of the method according to the invention, it is preferablyprovided that the first pulse signal and second pulse signal aregenerated as continuous pulse signal trains with constant frequency andthe control signal in the upper part-load range is generated from thetwo pulse signal trains by conducting an OR operation.

[0044] In the same way, there is also the possibility of generating thecontrol signal in the lower part-load range, the pulse width in thiscase being kept substantially at 0.

[0045] Further features and advantages of the invention are the subjectof the following description and the graphic representation of anexemplary embodiment, in which:

[0046]FIG. 1 shows a schematic representation of a first exemplaryembodiment of a control circuit according to the invention;

[0047]FIG. 2 shows a schematic representation of the division of thepart-load range into a lower part-load range and an upper part-loadrange;

[0048]FIG. 3 shows a schematic representation of the generation of apulse control signal within the scope of the solution according to theinvention from a first pulse signal train and a second pulse signaltrain by conducting an OR operation;

[0049]FIG. 4 shows an exemplary representation of the control signal inthe case of various part-load values within the scope of the solutionaccording to the invention;

[0050]FIG. 5 shows a schematic representation of the division of theupper part-load range into a normal upper part-load range and a highestupper part-load range;

[0051]FIG. 6 shows a schematic representation of a variant of the firstexemplary embodiment in which only one of the pulse signals ispulse-width-modulated at any time, while the other is notpulse-width-modulated;

[0052]FIG. 7 shows a schematic representation of the reduction ingenerated heat possible according to the invention;

[0053]FIG. 8 shows a representation of an actual control signal with thecorresponding current flowing via an inductive load, with a small pulsewidth of the first pulse signal, and

[0054]FIG. 9 shows a representation of an actual control signal with alarge pulse width and a corresponding current through an inductive loadwith a freewheeling diode.

[0055] An exemplary embodiment of a control circuit according to theinvention, represented in FIG. 1, comprises a load circuit 10, whichextends between a positive feed voltage U_(batt) and earth and in whichthere is a semiconductor circuit, which is designated overall by 12 andis formed for example as a MOSFET, a drain terminal D being connected tothe positive feed voltage U_(batt), a gate terminal G being provided fordriving purposes and a source terminal being connected to a load 14, forexample an inductive load in the form of a motor in particular.Connected in parallel with this inductive load 14 there is, furthermore,a freewheeling diode 16, so that the load 14 and the parallel-connectedfreewheeling diode 16 are on the one hand connected to the sourceterminal S of the semiconductor circuit 12 and on the other hand areconnected to earth.

[0056] For driving the semiconductor switch 12, in the case of theMOSFET semiconductor switch the gate terminal G is connected to a drivecircuit 20, which generates a control signal S which is supplied to thegate terminal G and comprises drive pulses following one after the otherand separated by interpulse periods. This control signal S is generatedby a pulse generator 22 and a following pulse-shaping stage 24, thepulse generator 22 generating square-wave pulses, the edge steepness ofwhich is fixed by the pulse-shaping stage 24, for example in such a waythat the rise and fall edges are delayed sufficiently long to give thefreewheeling diode 16 enough time for switching on or switching off.

[0057] The pulse generator 22 preferably comprises an OR gate 26, twopulse signal generators 28 and 30 and a control computer 32, whichdrives the pulse signal generators 28 and 30, so that the latter fortheir part generate pulse signal trains P₁ and P₂, respectively, whichare added in the adder 26 to form a pulse control signal PS.

[0058] The control computer 32 receives via an input signal E theinformation concerning the power with which the load 14 is to beoperated.

[0059] As represented in FIG. 2, there is an internal differentiation inthe control computer, on the basis of the input signal E, between alower part-load range UT and an upper part-load range OT, the lowerpart-load range UT preferably beginning at a power feeding of 0% withrespect to full load and reaching in the part-load range T up to a partload of, for example, 40% with respect to full load. This is thendirectly followed by the upper part-load range OT, which reaches up tofull power V, i.e. power of 100%, so that the entire part-load range Tis fully covered by the lower part-load range and the upper part-loadrange.

[0060] It is also conceivable, however, to choose the part-load range insuch a way that it neither begins at 0% nor ends at 100%, but liesbetween these values.

[0061] If the input signal E provides that the load 14 is to be operatedin the upper part-load range OT, the control computer 32 drives thefirst pulse signal generator 28 in such a way that it generates a pulsesignal train P₁ with a first frequency f₁, the individual pulses PE₁ ofwhich respectively start after time intervals Δt₁ which correspond tothe frequency f₁.

[0062] These individual pulses PE₁ have in this case a pulse width PW₁which can be set by the control computer 32, whereas, for the sake ofsimplicity, the frequency f₁ cannot be adjusted but instead can beprescribed as a fixed value for the first pulse signal generator 28.

[0063] The setting of the pulse width PW₁ takes place, as explained indetail below, in accordance with the power desired at the load.

[0064] Furthermore, the control computer 32 drives the second pulsesignal generator 30, which generates a second pulse signal train P₂,likewise represented in FIG. 3, the second pulse signal train P₂generating individual pulses which follow one another with a secondfrequency f₂ and are consequently generated one after the other in atime interval Δt₂ corresponding to this second frequency f₂.

[0065] In this case, the frequency f₂ is at least ten times thefrequency f₁; frequencies of the order of magnitude of 10 kHz or several10 kHz are preferably chosen for the frequency f₂, whereas thefrequencies f₁ are of the order of magnitude of some 100 Hz, so that, interms of its order of magnitude, the frequency f₂ is preferably onehundred times the frequency f₁.

[0066] The pulse width PW₂ of the individual pulses PE₂ of the secondpulse signal generator 30 can also be set by the control computer 32,whereas the frequency of the second pulse signal generator is usuallylikewise prescribed as a fixed value.

[0067] As represented in FIG. 3, the two pulse signal trains P₁ and P₂are then combined by a logical operation by the adder 26 or the OR gatein such a way that a pulse control signal PS which represents thelogical OR operation combining the two pulse signal trains P₁ and P₂ isobtained as a result, so that the pulse control signal PS exhibits theindividual pulses PE₁ of the first pulse signal train P₁, since theexistence of the individual pulses PE₂ of the second pulse signal trainP₂ is insignificant during the time in which the individual pulses PE₁are present, in particular whenever the individual pulses PE₁ have agreater pulse width PW₁ than the individual pulses PE₂ of the secondpulse signal train.

[0068] During the interpulse periods PP between the individual pulsesPE₁ of the first pulse signal train P₁, however, the individual pulsesPE₂ of the second pulse signal train P₂ then occur in the pulse controlsignal PS.

[0069] Since the pulse widths PW₁ of the individual pulses PE₁ of thefirst pulse signal train P₁ are generally greater than the pulse widthsPW₂ of the individual pulses PE₂ of the second pulse signal train P₂, alonger switching-through of the semiconductor switch 12 takes place eachtime the individual pulses PE₁ occur in the pulse control signal PS incomparison with the switching-through of the semiconductor switch 12when an individual pulse PE₂ from the second pulse signal train P₂occurs during the interpulse periods PP between the individual pulsesPE₁ of the first pulse signal train P₁.

[0070] In the upper part-load range OT, there is then the possibility ofcontrolling the power supplied to the load 14 by varying either thepulse width PW₁ of the individual pulses PE₁ of the first pulse signaltrain P₁ or the pulse width PW₂ of the individual pulses PE₂ of thesecond pulse signal train P₂ or the pulse widths PW₁ and PW₂ of bothpulse signal trains P₁ and P₂, so that the power to be supplied to theload 14 can be adjusted as finely and precisely as desired.

[0071] However, the upper part-load range OT is preferably operated insuch a way that the pulse width PW₂ of the individual pulses PE₂ of thesecond pulse signal train P₂ is kept as small as possible, by contrastwith the greatest possible pulse width PW₁ of the individual pulses PE₁of the first pulse signal train P₁, since the number of switching-on andswitching-off operations can be reduced with an increasing pulse widthPW₁ of the individual pulses PE₁ of the first pulse signal train if thepulse width PW₁ extends over at least one interval Δt₂ of the secondpulse signal train P₂ and the overall number of switching-on andswitching-off operations per unit of time decreases with an increasingpulse width PW₁ of the individual pulses PE₁ of the first pulse signaltrain P₁, since, on account of the switching-through of thesemiconductor switch 12, the load 14 is continuously fed for a longerand longer period of time within the time interval Δt₁.

[0072] On the other hand, in the lower part-load range UT, preferablyonly the pulse control signal PS is generated on the basis of the secondpulse signal train P₂ with variable pulse width PW₂.

[0073] Such a type of power control of the load 14 is indicated by wayof example in FIG. 4 at several points of the part-load range T.

[0074] For example, in FIG. 4a, the pulse width PW₂ in the case of apart-load range of 25% with respect to full load has been chosen suchthat it corresponds to a quarter of the time interval Δt₂ which liesbetween the beginning of two successive individual pulses PE₂ of thesecond pulse signal train. If, as represented in FIG. 4b, it is intendedto operate with a part load of 40%, the pulse width PW₂′ of the pulsecontrol signal, which is still composed of individual pulses PE₂ of thesecond pulse signal train P₂, is increased.

[0075] If, as represented in FIG. 4c, a transition is then made from thelower part-load range into the upper part-load range, which begins forexample at approximately 40%, the pulse control signal PS exhibits, asrepresented in FIG. 4c by way of example, on the one hand the individualpulses PE₂ with the pulse width PW₂ of the second pulse signal train P₂,but additionally the individual pulses PE₁ with the pulse width PW₁ ofthe first pulse signal train P₁, the individual pulses PE₂ of the secondpulse signal train P₂ only occurring in the interpulse periods PPbetween successive individual pulses PE₁ of the first pulse signal trainP₁.

[0076] Consequently, in the upper part-load range OT, both the pulsewidth PW₂ of the second pulse signal train P₂ and the pulse width PW₁ ofthe first pulse signal train P₁ can be controlled when increasing thepower to be made available to the load 14. The pulse width PW₁ of theindividual pulses PE₁ of the first pulse signal train is preferablyincreased with increasing power made available to the load 14, whereas avariation of the pulse width PW₂ of the individual pulses PE₂ of thesecond pulse signal train is only carried out for adjusting the power instages graduated as finely as possible.

[0077] The pulse width PW₁ of individual pulses PE₁ of the first pulsesignal train can be increased to such an extent that only one or just afew individual pulses PE₂ occur in the interpulse period PP betweensuccessive individual pulses PE₁.

[0078] To allow control to be carried out with as much precision aspossible near to full power, it is preferably provided that, in theproximity of full load, the power control is substantially carried outby adjusting the pulse width PW₂ of the individual pulses PE₂ of thesecond pulse signal train.

[0079] To allow the power to be controlled with as much precision aspossible in particular in the proximity of the full-load range, theupper part-load range is preferably divided, as represented in FIG. 5,into a normal upper part-load range NOT, which reaches for example from40% part load to 90% part load, and into a highest upper part-load rangeHOT, which reaches from 90% to 100%.

[0080] In this highest upper part-load range HOT, the pulse width PW₁ isno longer altered on the basis of the value of the pulse width PW₁ ofthe individual pulses PE₁ of the first pulse signal train at 90%, evenwhen there is an increase in power, but instead this is followed bycontrolling the power by means of the variation of the pulse width PW₂of the few individual pulses PE₁ of the second pulse signal train P₂ inthe interpulse periods PP between successive individual pulses PE₁, toallow this control to be performed with as much precision as possible.

[0081] In principle, both a variation of the pulse width PW₁ and avariation of the pulse width PW₂ would be possible here for setting thepower in the normal upper part-load range NOT.

[0082] This procedure is particularly simple in the normal upperpart-load range NOT whenever, on the basis of the transition from thelower part-load range UT into the upper part-load range OT, the maximumpulse width PW₂ is retained, this being the pulse width when there ismaximum power feeding of load in the lower part-load range UT, and thenthe individual pulses PE₁ with the pulse width PW₁ of the first pulsesignal train P₁ are additionally added, these then replacing one or moreindividual pulses PE₂ of the second pulse signal train P₂, the entirepower control in the normal upper part-load range preferably takingplace exclusively by means of the pulse width PW₁ with a constant pulsewidth PW₂′, until the highest upper part-load range HOT is reached.

[0083] A procedure of this type is represented in FIGS. 6a and b.According to FIG. 6a, the pulse width PW₂ in the lower part-load rangeUT is increased, beginning from a value substantially close to 0%, up tothe value PW₂′, at which a transition takes place from the lowerpart-load range into the upper part-load range, the power control takingplace in the normal upper part-load range, as represented in FIG. 6,exclusively by variation of the pulse width PW₁ until the highest upperpart-load range HOT is reached, the pulse width PW₁ then in turn beingkept constant in this range at the maximum value of the normal upperpart-load range, whereas the power control takes place exclusively bymeans of the pulse width PW₂′, which for this purpose is increasedbeyond the value PW₂′, which represents the maximum value at thetransition from the lower part-load range into the upper part-loadrange.

[0084] As represented in FIG. 7, the solution according to the inventionallows the thermal losses occurring overall in the case of the controlcircuit according to the invention to be reduced. FIG. 7 shows in theform of the curve A the progression of the heat losses when the entirepower control over the entire range takes place by means of the secondpulse signal train P₂ and the variation of the pulse width PW₂.

[0085] If, on the other hand, as represented by the curve B, the poweris controlled in the upper part-load range OT by a combination of theindividual pulses PE₁ of the first pulse signal train P₁ and theindividual pulses PE₂ of the second pulse signal train P₂, the heatlosses can be significantly reduced. The heat losses can be reduced evenmore significantly if the transition from the lower part-load range UTinto the upper part-load range OT is set at lowest possible values ofthe part load, as the curve C shows.

[0086] The actually occurring current I_(load) at the load 14 isrepresented in FIG. 8 for a case similar to FIG. 4c, i.e. for the casein which the pulse width PW₁ is so low that it extends over fewindividual pulses PE₂ of the second pulse signal train P₂. In this case,the current I_(load) flowing via the load 14 rises during the individualpulse PE₁ and then slowly falls, the current I_(load) changing inaccordance with the series of individual pulses PE₂.

[0087] If on the other hand, as represented in FIG. 9, the pulse widthPW₁ of an individual pulse PE₁ is chosen to be greater, the progressionof the current I_(load) over the load 14 is dominated primarily by theindividual pulses PE₁ with the pulse width PW₁ and only to a slightdegree by the individual pulses PE₂ in the interpulse periods of thefirst pulse signal train P₁.

1. Control circuit for the power-controlled operation of a load,comprising a semiconductor switch which is effective in a load circuitand comprising a drive circuit for the semiconductor switch, whichgenerates a control signal, comprising drive pulses following one afterthe other and separated by interpulse periods, for controlling the saidsemiconductor switch in a part-load range, characterized in that thecontrol signal (S) generates in an upper part-load range (OT) a firstpulse signal (P₁), with first individual pulses (PE₁) following oneanother with a first pulse frequency (f₁), and a second pulse signal(P₂), with second individual pulses (PE₂) following one another with asecond frequency (f₂), in the interpulse periods (PP) of the first pulsesignal (P₁), and in that the second frequency (f₂) is greater than thefirst frequency (f₁) by at least a factor of
 10. 2. Control circuitaccording to claim 1, characterized in that in the upper part-load range(OT) at least one of the first and second pulse signals (P₁, P₂) can bepulse-width-modulated for power control.
 3. Control circuit according toclaim 2, characterized in that in the upper part-load range (OT) onlyone of the pulse signals (P₁, P₂) can be pulse-width-modulated. 4.Control circuit according to claim 2 or 3, characterized in that theupper part-load range (OT) can be divided into a highest upper part-loadrange (HOT) and a normal upper part-load range (NOT) and in that theindividual pulses (PE₁) of the first pulse signal (P₁) can bepulse-width-modulated in the normal upper part-load range (NOT) and inthat the individual pulses (PE₂) of the second pulse signal (P₂) can bepulse-width-modulated in the highest upper part-load range (HOT). 5.Control circuit according to claim 4, characterized in that in thenormal upper part-load range (NOT) the pulse width (PW₂) of theindividual pulses (PE₂) of the second pulse signal (P₂) is constant. 6.Control circuit according to claim 4 or 5, characterized in that in thehighest upper part-load range (HOT), the pulse width (PW₂) of theindividual pulses (PE₂) of the first pulse signal (P₁) is constant. 7.Control circuit according to one of the preceding claims, characterizedin that, in a lower part-load range (UT) lying below the upper part-loadrange (OT), the control signal (S) comprises a third pulse signal (P₂)with a third frequency (f₂), which is greater than the first frequency(f₁).
 8. Control circuit according to claim 7, characterized in that thethird frequency (f₂) is of the same order of magnitude as the secondfrequency (f₂).
 9. Control circuit according to claim 8, characterizedin that the third frequency (f₂) and the second frequency (f₂) areapproximately of the same magnitude.
 10. Control circuit according toclaim 9, characterized in that the third frequency is identical to thesecond frequency (f₂).
 11. Control circuit according to one of claims 7to 10, characterized in that the third pulse signal (P₂) can likewise bepulse-width-modulated.
 12. Control circuit according to one of claims 7to 11, characterized in that the transition from the lower part-loadrange (UT) into the upper part-load range (OT) takes place at part-loadvalues in the range from approximately 20% to approximately 50%. 13.Control circuit according to claim 12, characterized in that thetransition from the lower part-load range (UT) into the upper part-loadrange (OT) takes place at part-load values in the range fromapproximately 30% to approximately 40%.
 14. Control circuit according toone of the preceding claims, characterized in that the second frequency(f₂) and/or the third frequency (f₂) is greater than the first frequency(f₁) by a factor of the order of magnitude of 30 or more.
 15. Controlcircuit according to one of claims 7 to 14, characterized in that theupper part-load range (OT) follows on directly from the lower part-loadrange (UT).
 16. Control circuit according to claim 15, characterized inthat the lower part-load range (UT) and the upper part-load range (OT)cover the entire part-load range up to full load.
 17. Control circuitaccording to one of the preceding claims, characterized in that thedrive circuit (20) has a pulse generator (22) and a pulse-shaping stage(24).
 18. Control circuit according to one of the preceding claims,characterized in that the first pulse signal (P₁) and the second pulsesignal (P₂) can be generated as pulse signal trains (P₁, P₂) havingindividual pulses (PE₁, PE₂) following continuously one after the otherwith constant frequency (f₁, f₂) and in that the control signal (S) forthe upper part-load range (OT) is produced from the pulse signal trains(P₁, P₂) by conducting an OR operation.
 19. Method for thepower-controlled operation of a load by means of a control circuit,comprising a semiconductor switch which is effective in a load circuitand comprising a drive circuit for the semiconductor switch, whichgenerates a control signal, comprising drive pulses following one afterthe other and separated by interpulse periods, for controlling the saidsemiconductor switch in a part-load range, characterized in that thecontrol signal (S) is generated in an upper part-load range (OT) in sucha way that it has a first pulse signal (P₁), with individual pulses(PE₁) following one another with a first pulse frequency (f₁), and asecond pulse signal (P₂), with individual pulses (PE₂) following oneanother with a second pulse frequency (f₂), in the interpulse periods(PP) of the first pulse signal (P₁), and in that the second pulsefrequency (f₂) is greater than the first pulse frequency (f₁) by atleast a factor of
 10. 20. Method according to claim 19, characterized inthat the power control is carried out by pulse width modulation of atleast one of the first and second pulse signals (P₁, P₂).
 21. Methodaccording to claim 20, characterized in that only the pulse width (PW)of one of the pulse signals (P₁, P₂) is modulated, whereas the other ofthe pulse signals (P₂, P₁) is kept constant
 22. Method according to oneof claims 19 to 21, characterized in that the upper part-load range (OT)is divided into a highest upper part-load range (HOT) and a normal upperpart-load range (NOT).
 23. Method according to claim 22, characterizedin that the first pulse signal (P₁) is modulated with regard to thepulse width (PW) for the power control in the normal upper part-loadrange (NOT).
 24. Method according to claim 22 or 23, characterized inthat the second pulse signal (P₂) is modulated for controlling the powerin the highest upper part-load range (HOT).
 25. Method according to oneof claims 22 to 24, characterized in that the other pulse signal (P₂,P₁), respectively, is kept constant with regard to its pulse width (PW).26. Method according to one of claims 19 to 25, characterized in that,below the upper part-load range (OT), a control signal (S) whichcomprises a third pulse signal (P₂) with a third frequency (f₂), whichis greater than the first frequency (f₁), is generated.
 27. Methodaccording to claim 26, characterized in that the third frequency (f₂) isof the same order of magnitude as the second frequency (f₂).
 28. Methodaccording to claim 27, characterized in that the third frequency (f₂) issubstantially identical to the second frequency (f₂).
 29. Methodaccording to one of claims 26 to 28, characterized in that the thirdpulse signal (P₂) is modulated with regard to its pulse width (PW₂). 30.Method according to one of claims 26 to 29, characterized in that thelower part-load range (UT) and the upper part-load range (OT) cover theentire part-load range up to full load.
 31. Method according to one ofclaims 19 to 30, characterized in that the first pulse signal (P₁) andthe second pulse signal (P₂) are generated as continuous pulse signaltrains (PS) with constant frequency and the control signal in the upperpart-load range (OT) is generated from the two pulse signal trains (P₁,P₂) by conducting an OR operation.